1. Technical Field
The present invention relates to cathodic protection systems for reinforced concrete structures, such as bridge and car park decks and piers. The present invention is particularly directed to locating exposed steel in the surface of the concrete. Such exposed steel can cause shorts in the cathodic protection systems.
2. Description of the Prior Art
In steel reinforced concrete structures, steel corrosion is the result of electrical current flowing from one point of the steel reinforcement to another. Such corrosion is enhanced by moisture and salt contamination of the concrete. Cathodic protection applies an external direct current to the steel reinforcement which counteracts the corrosive current.
U.S. Pat. No. 4,506,485 discloses one system for cathodic protection of reinforced concrete structures. The system comprises a coating of zinc metal which is flame-sprayed onto at least a part of the exposed concrete surface. The zinc metal coating is connected with the reinforcing steel through a power source by which current flow is induced from the metal coating to the reinforcing steel. By flame-spraying the metal onto the concrete, a coating is obtained which, in contrast to a metal coating painted onto the concrete and which contains a binder, is much more conductive. The flame-sprayed metal coating thus more effectively distributes the cathodic protection current to the reinforced concrete structure.
A publication entitled "Cathodic Protection of Reinforced Concrete Using Metallized Zinc", Materials Performance, Vol. 26, December, 1987, pages 22-28, discloses additional details of the system disclosed in U.S. Pat. No. 4,506,485.
A Federal Highway Administration Report entitled "Further Improvements in Cathodic Protection of Bridge Structures", Report FHWA RD-87/062, dated June, 1987, discloses subject matter closely related to the "Materials Performance" publication and to U.S. Pat. No. 4,506,485. On page 9 of the Report, it is confirmed that the zinc metallized coating has the advantage that it provides efficient current distribution. However, it is stated in the Report that the potential for electrical shorts, using a zinc metallized coating, is very high. These shorts are caused by exposed steel in the surface of the concrete. A short circuit can occur between the reinforcing steel and the metal coating. A short circuit can be caused by a tie wire for the reinforcing steel, used to tie the reinforcing bars together, having tie ends which extend into a crack in the concrete surface. A short circuit can also be caused by voids, such as cracks, crevices, potholes, or spalls in the concrete which expose a reinforcing bar or tie end. On page 49 of the Report, it is stated that prior to metal spraying, exposed steel should be located by visual inspection, and the surface scanned with a Fisher M-100 Rebar Locator. This is a magnetometer conventionally used to locate reinforcing steel. The magnetometer has a sensitivity which can be adjusted up or down. Because of the amount of steel in a conventional reinforced structure, the sensitivity of the magnetometer has to be turned down to the point where small wire ends from tie wires will not be detectable. On page 50 of the Report, reference is made to a test application in which, in a particular area, a total of eight shorts were found subsequent to metal spraying which were not located by visual inspection or use of a magnetometer. It is also indicated that these shorts were caused by protruding tie wire ends.
A California Department of Transportation report dated May, 1989 entitled "Development, Testing and Field Application of Metallized Cathodic Protection Coatings on Reinforced Concrete Substructures", Report No. FHWA/CA/TL-89/04, also discloses subject matter closely related to that of the "Materials Performance" publication and to U.S. Pat. No. 4,506,485. The report also confirms on page 93 that subsequent to sandblasting the concrete, the concrete should be electrically inspected to locate potential short circuits in the concrete.